Photosynthetic water oxidation is catalyzed by a cluster of 4 Mn atoms. Chloride and calcium are essential cofactors for oxygen evolution activity. It has been postulated that halide is a ligand to Mn, but the evidence at present is not conclusive. Cl K-edge studies by Hedman et al (Hedman, B., Hodgson, K.O. & Solomon, E.I. (1990) J. Am. Chem. Soc. 112, 1643-1645) have shown that there is a distinctive pre-edge feature in the K-edge spectrum of Cl when it is a ligand to transition metals. The feature was assigned to a Cl 1s to metal 3d transition, which becomes allowed due to mixing with empty d-orbitals. Such a pre-edge feature is present only when Cl- is ligated to a metal with empty d-orbitals. We propose to collect Cl K-edge spectra of photosystem II samples containing only the minimal amount of Cl-required (1Cl/4Mn) for activity to determine whether Cl is ligated to Mn.